1. Field of the Invention
The present invention relates to electronic circuits, and more specifically to methods and devices for generating a pulse signal with modulable-width pulses.
2. Description of Related Art
Pulse width modulation (PWM) signals, which are pulse signals with modulable-width pulses, have many applications. For example, such signals are used to drive power transistors that control the current flowing through an electric motor, such as those used in the automobile industry for controlling electric windows, sunroofs, and windscreen wipers. Conventionally, a set-point signal (generally with a "sawtooth" form) is generated using an oscillator, and is compared with a control voltage by a comparator. The output of the comparator then delivers the pulse signal, whose duty cycle is determined by the value of the control voltage.
In certain applications, either a continuous analog voltage or a logical signal with two states (0 or 1) can be used as the control signal (for example, to produce a pulse control signal whose two levels are 0 and 5 volts). In other words, it may be necessary to carry out "analog" PWM control or "logical" PWM control in certain applications. Although conventional circuits for generating PWM pulse signals operate correctly when the control signal is a continuous analog voltage, such conventional circuits have operating problems when a logical signal is supplied to the input of the comparator as the control signal. In particular, problems are caused by instabilities in the PWM signal when the two inputs of the comparator are equal (for example, both at 0 volts).